Estrogen (E) deficiency is a key cause of post-menopausal osteoporosis (PMO), yet the mechanism by which E deficiency results in bone loss remains unknown. There is increasing evidence that the T-cell may mediate bone loss. We hypothesize that bone loss occurring in PMO is a result of an adaptive immune response leading to T-cell activation/proliferation and increased thymic output of T-cells, ultimately leading to an expansion of T-cells producing TNF-alpha (TNF) and RANKL, two factors important for osteoclast formation. Our hypothesis is based on pre-clinical data that demonstrates that ovariectomy (ovx) in mice leads to enhanced T-cell production of TNF leading to bone loss whereas TNF-/- or nude mice are spared bone loss. Our pre-clinical data also suggests that there is increased thymic output of T-cells accounting for -50% of the bone loss occurring after ovx. To evaluate our hypothesis, we will conduct a clinical trial consisting of pre-menopausal women undergoing ovx with the following aims to be evaluated before and after ovx: 1) to determine the percentage of activated T-cells, proliferation state and phenotype of T-cells, 2) to quantify the amount of TNF and RANKL produced by the T-cell and its effect on up-regulating osteoclast formation, 3) to quantify the contribution of the thymus to increase the T-cell pool 4) to determine the effects of E therapy in reversing the T-cell activation/proliferation and its ability to reverse bone resorption by inhibiting T-cell production of TNF and RANKL. Peripheral T-cells will be collected from subjects before and after ovx. T-cell function will be evaluated by pre-osteoclast formation assays using T-cell conditioned media, flow cytometry for T-cell activation/proliferation markers and T-cell phenotype determination. The contribution of the thymus to ovx induced bone loss will be evaluated by thymic receptor excision circle (TREC) assay and measurement of thymus size by CT scan. T-cell and thymic function will be correlated with bone loss by serial bone turnover markers and bi-annual BMD determinations. The results are expected to confirm our previous pre-clinical research demonstrating the central role of the T-cell in E deficiency induced osteoporosis. Through a comprehensive career development plan under the supervision of an experienced mentor and an excellent advisory committee, this project will allow the applicant the necessary knowledge, skills and experience to develop into an independent investigator in patient oriented research in academia. [unreadable] [unreadable] [unreadable] [unreadable]